A roof of a building such as a house and a factory can be formed by installing a plurality of roofing materials each having a substantially rectangular shape on a roof base in a vertical direction and in a lateral direction sequentially. In a case of a roof having such a roof structure, respective sides of the two roofing materials adjacent to each other in a lateral direction of the roof, i.e. in a direction perpendicular to an inclining direction of the roof are overlapped each other (e.g. see document 1 [JP 08-277606 A]). FIG. 11 shows a partial cross-sectional view of one example of the above type of roof structure. Each of roofing materials 100 for this roof structure includes a cover part 40 having a substantially flat plate shape formed at one of opposite ends thereof and a water barrier part 41 having a wave shape in a sectional view formed at the other end thereof. The water barrier part 41 includes a plurality of protrusions 41a. The adjacent two roofing materials 100 are installed on a roof base 6 such that the water barrier part 41 of one of the adjacent two roofing materials 100 is covered by the cover part 40 of the other one of the adjacent two roofing materials 100. The adjacent two roofing materials 100 are installed under a condition that respective sides thereof are overlapped each other and thereby the above roof structure can improve a water barrier property of the roof and also has a water barrier function owing to the plurality of protrusions 41a formed on the water barrier part 41.
In a case where the respective sides of the two roofing materials are overlapped as mentioned above, when a lateral overlap between the sides is too small, rainwater may arrive at the roof base through the overlap and thus a water barrier property of the roof is likely to be insufficient. In contrast, when the lateral overlap between the sides is too long, the number of roofing materials 100 to be used is likely to increase, and therefore workability and appearance of the roof are likely to deteriorate. In view of this, in a process of overlapping the two roofing materials 100 and 100, it is necessary to adjust the length of the lateral overlap to a predetermined length. As a result, such adjustment of the length of the overlap may take troubles and time, and therefore workability may be insufficient. Further, precision of the adjustment of the length of the overlap may also strongly depend on a worker's experience, and thus precision of the installation may be unstable.